1. Field of the Invention
This invention relates to a switching power device and an inverter device, which are used as a dc stabilizing power source and in power transmission at noncontact points, and more specifically relates to a switching power transmission device for supplying energy from the secondary side of a transformer to a load by turning switching elements ON and OFF by means of half-bridge self-excited oscillation.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 1999-187664 (hereinafter “Patent Document No. 1”) discloses a conventional half-bridge switching power device using self-excited oscillation; in this device, a first switch circuit and a second switch circuit are connected to the primary side of a transformer, and a switching control circuit switches the switching elements in the switch circuits alternately ON and OFF before and after each period when they are concurrently OFF.
FIG. 1 shows a circuit diagram of this switching power device.
As shown in FIG. 1, in the switching power device, a primary winding T1 of a transformer T, an inductor L, and a first switch circuit S1 are connected in series to an input power E; a series circuit comprises a capacitor C and a switch circuit S2, and is connected in parallel between the primary winding T1 and the inductor L. A first drive winding T3 generates a voltage, which is approximately proportionate to the voltage of the primary winding T1, and connects to a first controller 11; similarly, a second drive winding T4 generates a voltage, which is approximately proportionate to the voltage of the primary winding T1, and connects to a second controller 12. The first controller 11 connects to a control terminal of a first switching element Q1 of the first switch circuit S1, and the second controller 12 connects to a control terminal of a second switching element Q2 of the second switch circuit S2. The first switch circuit S1 is a parallel-connected circuit comprising the first switching element Q1, a first diode D1, and a first capacitor C1. The second switch circuit S1 is a parallel-connected circuit comprising the second switching element Q2, a second diode D2, and a second capacitor C2.
A rectifying diode Ds is connected in series to a secondary winding T2 of the transformer T. A capacitor Co connects to the output side of the rectifying diode Ds, forming a rectifying-smoothing circuit. A capacitive impedance Cs is connected in parallel to the rectifying diode Ds, and a detector 14 for detecting load voltage is connected between the load and the output of the rectifying-smoothing circuit. The output from the detector 14 is fed back to the first controller 11.
By way of another example, Japanese Patent Application Laid-open No. 2002-112544 (hereinafter “Patent Document No. 2”) discloses a switching power device comprising a first controller 11, which controls the time taken from generating a voltage in the first drive winding T3 of the transformer T until a first switch circuit S1 turns ON and OFF, and a second controller 12, which controls the time taken from generating a voltage in the second drive winding T4 of the transformer T until a second switch circuit S2 turns ON and OFF, this switching power device operates self-excitedly by delaying the voltage from the drive windings by means of a delay circuit (using RC time constants), and controlling the ON/OFF times of the switching elements for a predetermined period of time.
However, the switching power devices described above have the following drawbacks.
The switching power device of Patent Document No. 1 is an ON/OFF device, which accumulates energy in the primary winding while the first switch circuit is ON, and discharges the accumulated energy from the secondary winding while the first switch circuit is OFF; however, as is clear from FIG. 1, since the sum of the input voltage Vin and the capacitor voltage Vc is applied to the switching elements Q1 and Q2, these elements must have high voltage resistance. Furthermore, since the input voltage Vin is applied directly to the primary winding T1 of the transformer T, the applied voltage is large, making it difficult to manufacture a smaller device.
The switching power device of Patent Document No. 1 comprises a first controller 11, which controls the times taken from generating a voltage in the first drive winding T3 of the transformer T until a first switch circuit S1 turns ON and OFF, and a second controller 12, which controls the times taken from generating a voltage in the second drive winding T4 of the transformer T until a second switch circuit S2 turns ON and OFF, this switching power device achieves self-excited operation by delaying the voltage from the drive windings by means of a delay circuit (using RC time constants), and controlling the ON/OFF times of the switching elements for a predetermined period of time. Therefore, self-excited operation can only be performed by using the voltages from the first and second drive windings T3 and T4, making it impossible to stabilize and control the operation with respect to variations among components and changes in temperature.